This invention relates generally to piezoelectric devices, and more particularly to heating piezoelectric devices through an excitation signal.
Piezoelectric devices, such as piezoelectric actuators, generally consist of a piezoelectric material that deforms when an electric field, e.g., a driving field, is applied across it. Additional materials may be bonded with the piezoelectric material, such as metallic layers that act as electrodes, insulating materials to prevent current from flowing between particular areas of the device, and adhesives to bond the various layers together.
In simplified terms, piezoelectric materials are comprised of many dipole unit cells. FIG. 1 symbolically depicts a unit cell 10 of a piezoelectric materia. When an electric field E1 is applied in the direction shown, the unit cell grows in the y axis and shrinks in the x axis, in essence becoming tall and thin. Conversely, when an electric field E2 is applied in the direction shown, the unit cell shrinks along the y axis and grows along the x axis, in essence, becoming short and fat.
As the unit cell 10 becomes colder, the piezoelectric effect, i.e., response to the application of an electric field, decreases. Thus, for a given magnitude of an electric field, the unit cell will not grow/shrink as much as it did when it was warmer. As a practical matter, for the same electric field applied, this decreases the stroke of the piezoelectric device. For example, for some piezoelectric materials, a 35% loss of stroke was found when the temperature changed from 25 degrees Celsius to xe2x88x9240 degrees Celsius.
The present invention provides apparatus and methods for exciting a piezoelectric device. A control device receives a first control signal excites the piezoelectric device at about at least one predetermined electrical resonant frequency of the piezoelectric device as a function of the first control signal.